Page 208 - Optical Communications Essentials

Wavelength Division Multiplexing

198 Chapter Twelve

next topic concerns the components needed for WDM realization. These range
in complexity from simple, passive optical splitting or combining elements to
sophisticated, dynamically tunable devices. The first component category
involves wavelength multiplexers, which are used to combine independent sig-
nal streams operating at different wavelengths onto the same fiber. The tech-
nologies for achieving this include thin-film filters (TFFs), arrayed waveguide
gratings (AWGs), Bragg fiber gratings, diffraction gratings, and interleavers.
The final topic concerns wavelength lockers which are important devices in
WDM transmitters to maintain the output from a laser diode at a predefined
ITU-T frequency with a high precision.
Chapter 13 on operational concepts shows how all the different component
puzzle pieces fit together to form a variety of metro and long-distance WDM links
and networks. Included there are discussions of WDM applications of dynamic
devices such as controllers for tunable transmitters, dynamic gain equalizers
(DGEs), tunable wavelength filters, and variable optical attenuators (VOAs).

12.1. Operational Principles of WDM

When optical fiber systems were first deployed, they consisted of simple point-
to-point links in which a single fiber line has one light source at its transmit-
ting end and one photodetector at the receiving end. In these early systems,
signals from different light sources used separate and uniquely assigned optical
fibers. In addition to filling up ducts with fibers, these simplex systems repre-
sent a tremendous underutilization of the bandwidth capacity of a fiber.
Since the spectral width of a high-quality source occupies only a narrow slice
of optical bandwidth, there are many additional operating regions across the
entire spectrum ranging from the O-band through the L-band that can be used
simultaneously. The original use of WDM was to upgrade the capacity of
installed point-to-point transmission links. This was achieved with wavelengths
that were separated from several tens up to 200nm in order not to impose strict
wavelength-tolerance requirements on the different laser sources and the
receiving wavelength splitters.
With the advent of tunable lasers that have extremely narrow spectral emis-
sion widths, one then could space wavelengths by less than a few nanometers.
This is the basis of wavelength division multiplexing, which simultaneously
uses a number of light sources, each emitting at a slightly different peak wave-
length. Each wavelength carries an independent signal, so that the link capacity
is increased greatly. The main trick is to ensure that the peak wavelength of a
source is spaced sufficiently far from its neighbor so as not to create interfer-
ence between their spectral extents. Equally important is the requirement that
these peak wavelengths not drift into the spectral territory occupied by adjacent
channels. In addition to maintaining strict control of the wavelength, typically
system designers include an empty guard band between the channels. Thereby
the fidelities of the independent messages from each source are maintained for
subsequent conversion to electric signals at the receiving end.

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